1. Field of the Invention
This invention relates to a method for controlling the water content of a catalyst stream comprising water and a strong acid. More specifically, this invention concerns a method for continuously monitoring the water content of an alkylation catalyst comprising water and an acid selected from the group consisting of sulfuric acid and fluorosulfuric acid by adding fuming sulfuric acid to a sample of said catalyst at a rate sufficient to maintain the mixture thus formed at the point of incipient fuming and detecting the presence of the SO.sub.3 evolved therefrom. The water content of the catalyst sample can then be determined from the flow rate of the sample and the flow rate and composition of the fuming sulfuric acid. The water content of the catalyst stream being monitored can then be controlled by regulating the addition rate of fresh acid to the alkylation process in accordance with the deviation between the actual water content and the desired water content.
2. Description of the Prior Art
The alkylation of isoparaffins with olefins in the presence of an acid catalyst such as sulfuric acid, fluorosulfuric acid and the like is well know in the petroleum refining art. Alkylation processes employing sulfuric acid have been extensively described in a number of publications (see, for example, "Petroleum Management", Vol. 33 (No. 13) pp. 203-215, Dec. 1961 and Vol. 34 (No. 1) pp. 207-217, Jan. 1962; "Petroleum Refiner", Vol. 37 (No. 9), pp. 316-329, Sept. 1958), the disclosures of which are incorporated herein by reference. Typically, in this process, an isoparaffin such as isobutane and olefins such as butenes are alkylated in the presence of a concentrated sulfuric acid catalyst which may range upward in strength from about 85 wt. % sulfuric acid. As the alkylation reaction proceeds, the strength of the acid catalyst, i.e. the activity, tends to decrease. This is due predominantly to dilution with water as well as to the formation of catalyst-hydrocarbon complexes. If the water content increases to too high a level, undesirable heavy alkylate will be produced. When the water content of the catalyst becomes very high, e.g., above about 5-6 wt. %, fresh acid makeup cannot be added to the system at a rate sufficient to maintain an acid strength conducive to stable operation. Furthermore, at such high water contents, the alkylation reaction ceases. We believe that these effects are due to the olefinic portion of the feed combining with the free sulfuric acid, i.e. the uncombined sulfuric acid, present in the catalyst to form dialkyl sulfates which preferentially accumulate within the hydrocarbon phase (due to their significantly higher solubility in the hydrocarbon relative to that in the acid) and thus deplete the acid catalyst inventory. If allowed to continue, substantially all of the catalyst inventory will be depleted. The acid (catalyst) phase remaining in the reaction zone will be predominantly monoalkylsulfates, water, carbonaceous complexes along with small amounts of dialkyl sulfates and sulfuric acid. This is not an alkylation catalyst. Therefore, it is desirable to maintain the acid strength above a certain minimum concentration relative to the water present in the catalyst to obtain a satisfactory operation. Should the acid strength drop below the established minimum, the catalyst inventory must be withdrawn from the reaction zone and replaced with fresh acid makeup.
In commercial operations, one method for determining and eventually controlling the free acid strength is by periodically measuring the titratable acidity of the acid stream, be it a fresh, intermediate or a spent acid stream. However, any monoalkyl sulfates present therein will titrate as equivalent to 0.5 of sulfuric acid so as to mask the trueamount of water present therein. Therefore, the measured free sulfuric acid strength will appear to be higher than the actual free sulfuric acid strength. Thus, the actual water/sulfuric acid ratio will be greater than indicated by this measurement. Another method for determining the free acid strength is to measure the amount of carbon and water in the acid stream since the wt. % free sulfuric acid will be equal to about 100 -- wt. % carbon -- wt. % water. Such measurements are normally done by one or more methods that are well known by one skilled in the alkylation art. However, either of the above methods for determining acid strength of the catalyst is time consuming and expensive when carried out in the laboratory and cannot be accomplished with sufficient rapidity for close operational control of the process.
When fluorosulfuric acid is employed as the alkylation acid, such as is described in U.S. Pat. No. 3,887,635, the disclosures of which are incorporated herein by reference, it is preferred to have essentially no free water present, although some may be present. However, the analytical techniques mentioned above may be used to determine the equivalent water present (i.e. the water equivalent to adding water, other oxygenated compounds or mixtures thereof to fluorosulfuric acid), but suffer from the deficiencies noted above.
It has also been suggested that the water content of a sulfuric acid catalyst can be determined by mixing fuming sulfuric acid with the acid catalyst until fuming is initiated (see Albright, L. F. et al, "Alkylation of Isobutane with Butenes: Effect of Sulfuric Acid Compositions", Ind. Eng. chem. Process Des. Devlop. Vol. 11 (No. 3), p. 446-450, 1972). However, neither this method nor any of the above-mentioned methods have been used to control continuously the water content of the acid catalyst by continuously controlling the addition rate of fresh acid to the alkylation process.